The use of electrotherapy by medical investigators historically reaches back to the eighteenth century. In that era, electrotherapy static generators were the subject of substantial interest. As the twentieth century was approached, experimentation applying high frequency currents to living tissue took place, d'Arsonal being considered the first to use high frequency currents therapeutically. The use of high frequency currents for the purpose of carrying out electrosurgical cutting and the like was actively promoted in the 1920s by Cushing and Bovie. In the 1970s, solid state electrosurgical generators were introduced, and a variety of such generators now are available in essentially all operating theatres.
When high frequency currents are used for cutting and coagulating, the tissue at the surgical site is subjected to controlled damage. Cutting is achieved by disrupting or ablating the tissue in immediate apposition to the excited cutting electrode, i.e., slightly spaced before it so as to achieve the formation of a cutting arc. Continuous sine waveforms generally are employed to carry out the cutting function where tissue cells adjacent to the electrode are vaporized. An advantage of this electrosurgical cutting procedure over the use of the cold scalpel resides both in an ease of cutting and a confinement of tissue damage to very small and shallow regions. In the latter regard, cells adjacent the cutting electrode arc are vaporized and cells only a few layers deeper are essentially undamaged. These cutting systems, in general, are employed in a monopolar manner wherein the cutting electrode is considered the active one and surgical current is returned from a large, dual component dispersive electrode coupled with the skin of the patient at a remote location.
Coagulation also may be carried out using a high frequency generator source and is accomplished by denaturation of tissue proteins due to thermal damage. Interrupted or discontinuous waveforms typically are employed to carry out coagulation. Coagulation is considered generically as including:                (1) fulguration in which tissue is carbonized by arc strikes,        (2) desiccation in which the cells are dehydrated, and        (3) white coagulation in which tissue is more slowly heated to a coagulum.The interrupted wave based coagulation procedure has been carried out with both monopolar and bipolar systems.        
In order to obtain cutting with hemostasis to arrest bleeding, present day electrosurgical generators may be controlled to blend cutting and coagulating waveforms. To achieve this blend, for instance, a lower amplitude continuous sine waveform is combined with higher amplitude coagulate pulses prior to output voltage elevation by power amplification procedures or the like.
The electrosurgical cutting reaction has been the subject of considerable study. In this regard, some investigators observed that cutting is achieved as the electrical conduction of current heats the tissue up to boiling temperatures and the cells are basically exploded as a result of the phase change. Another, parallel mechanism has been described wherein, as an intense electromagnetic field impinges on absorbing tissue, an acoustic wave is generated by the thermal elastic properties of the tissue. The origin of the pressure wave lies in the inability of the tissue to maintain thermodynamic equilibrium when rapidly heated. See generally:                                    “Electrosurgery” by J. A. Pierce, John Wiley & Sons New York, N.Y.                        
Paramount to the cutting procedure is the generation of an arc within the evoked vapor phase. When cutting is being performed, the cutting electrode is not in mechanical contact with tissue, but rather rides on a vapor film as it is moved through the tissue. Thus, it is the separation between the cutting electrode and tissue which allows the possibility for arc formation while cutting. With the existence of this arc, current flow is highly confined, arcs by their nature being quite localized in both space and time, consisting of very short high current density discharges.
Electrosurgical generators generally are configured to derive a requisite arc formation with an active electrode of fixed geometry. For instance, the active electrodes may take the shape of a rod or spade-shaped scalpel. Arc formation requires technique on the part of the surgeon, the electrode being gradually moved toward target tissue until the spacing-based impedance is suited for striking an arc. The energy creating the arc typically is generated by a resonant inverter operating at an RF frequency. Control over such inverters is problematic, inasmuch as the arc represents a negative dynamic impedance. In general, some regulation of voltage feeding the RF invertors is carried out, however, overall output control is based upon a power level selection. Inverter control by output voltage feedback generally has been avoided due principally to the above-noted load characteristics of the necessary arc. Such attempted control usually evolves an oscillatory instability. Accordingly, power-based control is employed with marginal but medically acceptable output performance.
Currently developing electrosurgically implemented medical instrumentation, however, has called for active cutting electrodes of highly elaborate configuration with a geometry which alters in active surface area during a procedure. Generators exhibiting a relatively constant power output cannot sustain an arc under such operational conditions. In this regard, the power output must be variable to track the changing shape and size of the active electrode. This, in effect, calls for an electrosurgical generator capable of producing an RF cutting output under constant voltage control and variable power conditions.
Another developing operational requirement for the electrosurgical generator is a concern for initial arc formation. Applications of the newly contemplated systems call for arc start-up when the active electrode is embedded within and in contact with the tissue to be cut. No preliminary impedance defining spacing otherwise attained by the technique of the surgeon is available to achieve initial arc formation.